Field of the Invention
The present invention pertains generally to the field of the electrical stimulation of muscles for prevention of thrombosis and for pain management and, more particularly, to electrical stimulation of muscles of the foot.
Description of the Related Art
Electrical stimulation of muscles and nerves by applying electrodes over the skin is currently used for enhancing blood circulation and reducing blood clots and for scrambling the pain signal that reach the brain in order to manage pain.
Patients undergoing surgery, anesthesia and extended periods of bed rest or other inactivity are often susceptible to a condition known as deep vein thrombosis, or DVT. DVT is a clotting of venous blood in the lower extremities or pelvis. This clotting occurs due to the absence of muscular activity required to pump the venous blood in the lower extremities, local vascular injury, or a hypercoagulable state. The condition can be life-threatening if a blood clot migrates to the lung, resulting in pulmonary embolism (PE), or otherwise interferes with cardiovascular circulation. More generally, venous thromboembolic disease (VTED) is a cause of significant morbidity and mortality for individuals immobilized after orthopedic surgery, due to neurologic disorders, even during prolonged travel, and a variety of other conditions.
Since 1954, it has been known that prolonged dependency stasis, a state imposed by airplane flights, automobiles trips and even attendance at the theater may bring on thrombosis. In 1977, it was shown that trips as short as three to four hours could induce DVT and PE.
DVT and related conditions may be controlled or alleviated by assisting blood circulation (venous return) in the muscles.
Current approaches to prophylaxis include mechanical compression using pneumatic compression devices, anticoagulation therapy and electrical stimulation of the muscles. Pneumatic compression equipment is often too cumbersome for mobile patients, or during prolonged travel. Anticoagulation therapy carries the risk of bleeding complications and must be started several days in advance to be effective. Electric stimulation has advantages over the other two methods in that it can be started at the time prophylaxis is needed and can be portable using DC current sources.
A number of U.S. patents teach various methods of applying electrical stimulation to the calf muscle for the prevention of DVT. These include Powell, III, U.S. Pat. No. 5,358,513; Tumey, U.S. Pat. No. 5,674,262; Dennis, III, U.S. Pat. No. 5,782,893; Katz, U.S. Pat. No. 5,643,331; and Katz, U.S. Pat. No. 6,002,965.
U.S. Pat. No. 6,615,080 to Unsworth et al. provides a method for preventing DVT, PE, ankle edema and venostasis and a device that includes a single channel sequential neuromuscular electrical stimulation (NMES) unit. The NMES unit is battery powered and can be programmed to deliver a particular stimulus profile. In order to simplify the patient's ability to properly apply the NMES device, the stimulator generates biphasic symmetrical square wave pulses with stimulus parameters demonstrated to result in optimum venous blood flow. The stimulus profile included a stimulus frequency fixed at 50 pulses per second, a stimulus duration of 300 microseconds, a ramp up time of 2 seconds, a ramp down time of 2 seconds, and a stimulus cycle set at 12 seconds on and 48 seconds off. Once set in advance by the doctor, manufacturer or user, the patient adjusts the intensity, using a stimulus intensity dial, to the point needed to produce a minimally visible or palpable muscle contraction. The output leads of the stimulator are attached through a conductor to electrodes of various types including, self-adherent surface electrodes. These electrodes are of opposite polarity and create an electrical potential difference between themselves and the tissue that separates them. The frequency and electrical characteristics of electrical impulses applied to the patient is referred to as the electrical stimulation routine.
In published but abandoned U.S. Patent Application Publication No. 2006/0085047 A, a variation of Unsworth et al. provided a method of automatically controlling the delivery of single channel NMES of the plantar muscle, in response to the sensing of motion of the foot or leg. In the published application, the stimulation is turned off during walking or running to prevent slips or falls and to reduce power consumption of the unit that provides the stimulation.
FIG. 1A, FIG. 1B, FIG. 1C, FIG. 1D, and FIG. 1E show muscles of the sole of a foot.
There are four layers of muscles in the sole of the foot. After the skin of the plantar region and the fatty tissue have been removed, an expansion of fibrous tissue known as the plantar fascia is visible. If this is also taken away, the first layer of muscles is exposed, consisting of abductor pollicis (14), flexor brevis digitorum (18), and the abductor minimi digiti (16) (FIG. 1A). The second layer, situated under the first, consists of the tendons of the flexors longus digitorum (11), proprius, and pollicis (12). On the outer side of the foot, the tendon of the peroneus longus (5) passes beneath the flexor accessories (20). To complete the layer, the muscles flexor accessorius (20) and the lumbricales (19) must be named (FIG. 1B). The third plantar layer consists of the tendon of tibialis posticus (10), the flexor brevis pollicis (15), the adductor pollicis (21), the flexor brevis minimi digiti (17), and, running across the foot, the transversus pedis (22). The sheath of the peroneus longus (5), and the plantar ligament, are also found in this layer (FIG. 1C). The fourth layer (FIG. 1F) consists of three interossei (23), one on the inner side of the second toe, and the others each on the inner side of the third and fourth toes.
They draw to the central line XY, called the “central muscular action line,” or the “line of muscular action.” The first layer (FIG. 1E) on the dorsal surface consists of the tendons of the tibialis anticus (1), extensor proprius pollicis (2), extensor longus digitorum (3), and the tertius peroneus (4). The muscles of the extensor brevis digitorum (13), after passing under the extensor longus digitorum (3), divide into four tendons, and aid in the extension of the toes. The second layer (FIG. 1D) consists of four interossei (23a), fixed on the outer side of the second, third, and fourth toes and drawing from the “central muscular action line” XY, and one on the inner side of the second toe drawing to line XY.
Muscles of the foot are also divided into a plantar group (internal, external, and central), which pertains to the sole of the foot, and a dorsal group, which indicates the back muscles behind the plantar muscles.
The dorsal group includes:
13. Extensor brevis digitorum. First layer.
23a. Interossei dorsal (4). Second layer.
The plantar group includes:
14. Abductor pollicis. Internal first layer.
15. Flexor brevis pollicis. Internal third layer.
16. Abductor minimi digiti. External first layer.
17. Flexor brevis minimi digiti. External third layer.
18. Flexor brevis digitorum. Central first layer.
19. Lumbricales. Central second layer.
20. Flexor accessorius. Central second layer.
21. Adductor pollicis. Central third layer.
22. Transversus pedis. Central third layer.
23. Interossei plantar (3). Fourth layer.
The location and function of each muscle is further described below.
13. The extensor brevis digitorum arises in the upper outer side of the heel-bone, and, broadening out, it passes under the extensor longus digitorum, when it divides into four tendons that go forward and are inserted in the bases of the first phalanges. Its action is to aid the extension of the toes and to counteract the tendency of obliquity of the extensor longus digitorum.
14. The abductor pollicis arises on the inner posterior region of the os calcis, and is inserted in the first phalanx of the great toe. Its action is to abduct the big toe away from the central line of the foot to the imaginary line that forms the centre of the body. By this action, the great toes would be brought closer together.
15. The flexor brevis pollicis comes from the second row of the tarsus, and is inserted to the base of the first phalanx.
16. The abductor minimi digiti arises from the outside of the os calcis, and goes forwards to the external side of the first phalanx of the little toe. Its action is to draw the little toe away from the middle line of the foot.
17. The flexor brevis minimi digiti has origin in the sheath of the peroneus longus and the base of the fifth metatarsal bone, and is inserted in the first phalanx of the little toe. Its action is to flex the little toe.
18. The flexor brevis digitorum, from the heel-bone and the plantar fascia, draws down the toes, and is inserted in the second phalanges of the four toes.
19. The four lumbricales are affixed to the inner side of the four toes. Their action is to draw the toes into the inner side of the foot.
20. The flexor accessorius extends from the os calcis to the second, third, and fourth toes. In contraction, it counteracts the obliquity of the flexor longus digitorum, hence its name.
21. The adductor pollicis arises from the sheath of the peroneus longus and the third and fourth metatarsals, and is inserted in the first phalanx of the great toe on the outer side. Its action is to adduct, or draw, the great toe to the central line of the foot.
22. The transversus pedis goes across the foot, and is inserted in the phalanx of the great toe. Its office is to adduct, or draw, the big toe to the line of the foot termed the “line of muscular action.”
23. The three plantar interossei are situated between the bones of the toes on the inner side, and draw to the central line the three outer toes.
23a. The four interossei, on the dorsal surface of the foot, are situated on the outer side of the bones of the toes, and draw the third and fourth toes away from the central line of muscular action. The two interossei on either side of the second toe draw away from the axis of the toe either to the outer or inner side of the foot, respectively.
The foot is provided with two kinds of nerves—those that supply the skin with sensory branches, and the other sort that give motor impressions to the muscles. The posterior tibial and the anterior tibial nerves come from the sciatic nerve, the former giving branches to the muscles in passing down to the inner side of the ankle. The posterior tibial then divides into external plantar nerves and internal plantar nerves, that supply the toes and sole of the foot. The anterior tibial nerves supply the dorsum of the foot as well as the outer side of the leg.
Under the skin are found pads of fat, at the heel and toes especially.
The muscles of the foot are further classified as either intrinsic or extrinsic. The intrinsic muscles are located within the foot and cause movement of the toes. These muscles are flexors (plantar flexors), extensors (dorsiflexors), abductors, and adductors of the toes. Several intrinsic muscles also help support the arches of the foot. The extrinsic muscles are located outside the foot, in the lower leg. The powerful calf muscle is among them. Most of these muscles have long tendons that cross the ankle, to attach on the bones of the foot and assist in movement.
FIG. 2 shows the flexor digitorum brevis muscle.
This muscle is responsible for flexing the four smaller toes. It lies in the middle of the sole of the foot, immediately above the central part of the plantar aponeurosis, with which it is firmly united. Its deep surface is separated from the lateral plantar vessels and nerves by a thin layer of fascia. It arises by a narrow tendon, from the medial process of the tuberosity of the calcaneus, from the central part of the plantar aponeurosis, and from the intermuscular septa between it and the adjacent muscles. It passes forward, and divides into four tendons, one for each of the four lesser toes.
Of the other muscle of the first layer, the abductor digiti minimi (abductor minimi digiti, abductor digiti quinti) is a muscle which lies along the lateral border of the foot, and is in relation by its medial margin with the lateral plantar vessels and nerves. Its function is to flex and abduct the fifth (little) toe. The last muscle of the first layer, abductor pollicis is like the abductor digiti minimi except that it lies along the lateral inside border of the foot and connects to the big toe.
FIG. 3A and FIG. 3B show placement of electrodes as disclosed by Unsworth et al., U.S. Pat. No. 6,615,080.
FIG. 3A illustrates a sole of a foot 31. Toes 32, ball 33, arch 34, and heel 35 are shown in the drawing. Electrodes 36a, 36b are located in an area over intrinsic muscles on the plantar surface of the foot, or proximal to them, for example on or around the ball of the foot 33, and over or proximal to the heel 35. In FIG. 3A, electrodes 36a and 36b are placed that deliver the electrical impulses generated by the NMES device 30. FIG. 3B shows an alternate area 36a′ at which an electrical impulse can be delivered. In some embodiments of the Unsworth invention, the electrode 36a occupies only the area of the ball of the foot, while other embodiments include elliptical electrodes having their major axis normal to the longitudinal axis of the foot 31.
As shown in FIG. 3A and FIG. 3B, the Unsworth issued patent applies one electrode over or proximal to the heel and the other over the intrinsic muscles on the plantar surface of the foot, for example, on or around the ball of the foot. In Unsworth, the intensity of the electrical stimulation required is only that necessary to create a slight visible muscle twitch of the foot muscles, or a minimally visible or palpable muscle contraction. By stimulating in this manner, blood pooling in the calf veins was prevented.
Electrical stimulation is also utilized for pain management. The most common form of electrical stimulation used for pain management is transcutaneous electrical nerve stimulation (TENS) therapy, which provides short-term pain relief. Electrical nerve stimulation and electrothermal therapy are used to relieve pain associated with various conditions, including back pain. For example, intradiscal electrothermal therapy (IDET) is a treatment option for people with low back pain resulting from intervertebral disc problems. In TENS therapy for pain management, a small, battery-operated device delivers low-voltage electrical current through the skin via electrodes placed near the source of pain. The electricity from the electrodes stimulates nerves in the affected area and sends signals to the brain that “scramble” normal pain perception. TENS is not painful and has proven to be an effective therapy to mask pain.